Gas inflated seal for a floating roof storage tank



Feb. 4, 1964 1. L. WISSMILLER GAS INFLATED SEAL FOR A FLOATING ROOFSTORAGE TANK Filed Nov. 27, 1959 2 Sheets-Sheet 1 Feb. 4, 1964 I. 1..WISSMILLER GAS INFLATED SEAL FOR A FLOATING ROOF STORAGE TANK 2Sheets-Sheet 2 Filed NOV. 27, 1959 IV yaw: 07v

United States Patent 3,120,320 GAS INFLATED SEAL FOR A FLGATING ROOFSTORAGE TANK Ivan L. Wissmiiler, Chicago, Ilh, assignor to (EhrcagoBridge & Iron Company, a corporation of Illinois Filed Nov. 27, 1959,Ser. No. 855,788 1 Claim. (Cl. 220-25) This invention relates to animproved type of seal for a floating roof tank for the storage ofvolatile products.

For many years, one of the most efficient storage con tainers forvolatile liquid products has been the flat bottomed cylindrical walledfloating roof type of tank that is well known in the art. In such a tankthere is generally provided a circular roof having a diameter somewhatless than the diameter of the cylindrical tank which is adapted to floatupon the surface of the liquid stored in the tank. A suitable sealingdevice of annular shape is arranged about the periphery of the floatingroof within the shell of the tank for the purpose of sealing the annularspace de fined by the rim of the floating roof and the inner surface ofthe tank shell against the escape of volatile fractions of the storedproduct. It is not economically feasible to construct the floating roofof exactly the same diameter as the inner surface of the cylindricalshell so as to fit together as a piston fits in an automobile cylinderbecause in a tank of the size under consideration the cylindrical shellis of relatively thin plate material which is subject to localdistortion caused by such factors as thermal shrinkage of welded joints,Wind forces, uneven foundation settlement and fabrication and erectiontolerances. For this reason the diameter of the floating roof may befrom twelve to twenty-four inches less than the diameter of the tankshell, and in such cases the annular space between the rim of the roofand the shell presents a significant source of evaporation loss if notproperly sealed.

According to the practices well known in the art, sealing devices forthe annular space between the floating roof rim and the tank shell havebeen of the mechanical or liquid tube type. In the mechanical type,relatively thin flexible metal shoes are placed in sliding contact withthe tank shell, supported by various mechanical linkages from the tankroof, with a curtain of seal material extending from the rim of the tankroof to the metallic shoes. This type of sealing device has certaininherent disadvantages, among them being corrosion and abrasion ofmetallic parts, distortion of the flexible metal shoes causingirnperfect contact with the tank shell with resulting high evaporationloss of the stored volatile product, and sparking resulting fromelectrical charges arcing across various metallic components thusincreasing the hazard from fires if not properly protected.

On the other hand, the liquid filled tube type of sealing device knownto the art, which in its basic form comprises a rubberized fabric sealmaterial extending across between the roof and the tank shell anddistended by means of a liquid filled annular tube, also presentscertain inherently undesirable characteristics. For example, thepressure exerted by an adequate depth of liquid may cause excessiveabrasion of the fabric material in sliding contact with the tank shell.Furthermore, it frequently happens that the weight of the liquid causesthe seal to sag rather than distend outward against the tank shell.

In accordance with the instant invention there is provided a gasinflated rubberized fabric type sealing device for a floating roof tankthat is not subject to the difliculties outlined above with respect toconventional types of sealing devices. The invention also provides asource of constant gas pressure within the seal irrespective ofvariations in temperature of the tank or its surrounding atmosphere andalso irrespective of a moderate loss of gas by 3,120,329 Patented Feb.4, 1964 diffusion through the fabric and by leakage. Additionaladvantages of the invention will become apparent in the detaileddisclosure which follows.

In a preferred embodiment of the invention, there is provided a floatingroof tank with a fabric seal extending from the roof rim to the insideor surface of the tank shell which is held in place by means of aflexible tube containing a gas such as air under a controlled pressure,together with means for maintaining the pressure within said tuberegardless of local temperature variations or other conditions whichotherwise might alfect the pressure and the performance of the sealingdevice.

The invention will be better understood from the following detaileddescription thereof taken in conjunction with the accompanying drawings,in which:

FIGURE 1 is a vertical cross-sectional view of a flat bottomedcylindrical liquid storage tank with a pontoon type floating roof and .asealing device constructed in ac cordance with the preferred embodimentof this invention;

FIGURE 2 is a similar view, on an enlarged scale, of a portion of thetank shell, the pontoon portion of the roof and the related sealingdevice with the pressure control means positioned in its normaloperating position;

FIGURE 3 is a similar view of the same device with the pressure controlmeans in its most extended or venting position;

FIGURE 4 is a similar view of the same device at a time when thepressure regulating means is in its most retracted or demand position;and

FIGURE 5 is a vertical cross-sectional view of another embodiment of theinvention which makes possible the elimination of a gasholder that isincluded in the preferred embodiment.

In FIGURES l and 2 there is shown a cylindrical tank having a flatbottom It} and vertical cylindrical sidewalls 11, within which ispositioned a floating roof 12 supported by the stored liquid. In thesefigures the floating roof is of the pontoon type, having a centralsingle deck 13a and a plurality of pontoons 13 arranged annularly aboutthe single deck so as to provide buoyancy for the roof even when greatamounts of rain water may have accumulated thereon.

As shown more clearly in FIGURE 2, an annular tube 14 completelyencircles the rim 15 of the floating roof and, when inflated, bearsagainst the interior surface of the tank shell 11. A connection 16 tothe tube 14 is provided as a source for supplying gas under pressure tothe tube.

In the preferred embodiment shown in FIGURE 2, a gasholder 17 is locatedinside one of the pontoons 13 and is connected by a suitable conduit 18to the connection 16 supplying gas to the tube 14. This gasholderconstitutes a reservoir for the storage of a supply of gas undersuitable pressure to accommodate the change in volume of the gasresulting from thermal contraction and expansion. The gasholder 17consists essentially of a sump 17a and a cover 19, said sump and coverbeing sealed to each other by a flexible diaphragm 20. The diaphragm 20permits the cover to rise and fall within rather wide limits toaccommodate varying volumes of gas stored in the gasholder 17. Theweight of the cover 13 is selected so as to place the gas stored withinthe gasholder at the optimum pressure desired for maintaining theltube14 in proper sliding relationship with the tank shel 11.

A suitable inlet valve 21 permits replacement of gas within thegasholder, and in the preferred embodiment this valve 21 is actuatedmechanically to open when the roof 19 is near its lowest operatingposition. Also provided in this embodiment is a venting valve 22connected by means of a suitable conduit 23 to the gasholder 17 and a toconnection 16, and in the preferred embodiment the venting valve will besupplied with mechanical means for opening when the gasholder covernears its fully inflated position.

As shown in FIGURES 2, 3 and 4, the automatic means for actuating thegas supply valve 21 and the venting valve 22 may be quite simple. Inthese figures the actuating means for both valves consist merely ofvertical rods 21a and 22a extending from the operating mechanism of thevalve downwardly to a selected point of contact with another member. Inthe case of the supply valve 21, this vertical control rod 21a isactuated by contact with the bottom of the gasholder, which in thisembodiment is also the bottom of the pontoon in which the gasholder islocated. The actuating means for the venting valve comprises a verticalcontrol rod 22a positioned to be contacted by the cover 19 shortlybefore the roof reaches its upper limit of travel. While this is apreferred embodiment, it should be obvious to anyone skilled in the artthat it is not the only method of achieving automatic gas supply valveopening and closing and gas venting valve opening and closing. In fact,these valves could be operated manually rather than automatically,although that is not the preferred method.

The cover 19 of the gasholder 17 shown in FIGURE 2 is in a positionwithin the range of normal operation, at which time there is asufiicient supply of gas within the gasholder to maintain the desirablepressure for most effective operation. In such position, both the supplyvalve 21 and the venting valve 22 are closed.

FIGURE 3 shows the condition of the gasholder cover 19 when thegasholder is nearly full. In this position cover 19 has become elevatedto its extreme uppermost position and has thereby actuated the controlrod 22a for venting valve 22. In this position, therefore, the ventingvalve is opened and gas from gasholder 17 and/or tube 14 is releasedinto the atmosphere, thus releasing excess gas without substantiallyincreasing the pressure above the optimum required in the tube 14.

FIGURE 4 shows the other extreme position, in which the gas within thesystem has reached such a low pressure that the cover 19 has fallen toits lowest operating position. In this position the gas supply valve 21is automatically opened by control rod 21a, thus permitting gas from aconvenient available supply (not shown) to enter the system, raising thecover 19 of the gasholder to a level within the range of normaloperation and maintaining the proper gas pressure for etficientoperation of the system.

In this preferred embodiment the gasholder maintains the properoperating pressure within the tube 14 despite tendencies for thepressure to increase or decrease because of breathing caused by suchfactors as daily temperature variations, changes in climatic conditions,stretch ing or other dimensional changes in the tube, moderate loss ofgas from diffusion or leakage, and the like. The capacity of thegasholder should be at least equal to the dilference between the maximumand minimum volumes occupied by the gas contained within the system.

By way of an example of the method of operation of this preferredembodiment, a synthetic rubber impregnated fabric tube 14 is selected ofsuch quality and dimensions that the desirable inflating pressure rangefor the tube which will permit maintaining the tube at all times insliding contact with the shell 11 without locking the roof into placeand inhibiting sliding, is 1 to 3 inches of water. A gasholder cover isselected having a-total weight (including appurtenances) that createsthis level of pressure in the gasholder. A sufficient supply of gas isthen let into the gasholder to inflate the tube to the desired pressure,which may be of the order of approximately 2 inches of water, and toprovide a reserve supply within the gasholder at a time during the daywhen the average temperature of the system is, for example, 70 F. It maybe expected that, after the sun has set and nocturnal cooling to about50 F. has taken place,

the volume of gas within the system at the established pressure isreduced by a substantial amount, thus permitting the cover to descendtowards its low operating position. No additional gas, however, need besupplied to the system to maintain its pressure unless the volume isdecreased to such an extent that the cover 19 descends to its lowestposition and actuates supply valve 21. As the next day progresses, thetemperature may increase until in the early afternoon the averagetemperature within the system may be as high as 100 F., at which timethe gas at the selected pressure will have expanded so as to raise thecover 19 to a much higher level without increasing the pressure. If thisexpansion raises the cover to its highest operating position, then theventing valve 22 will automatically be actuated to vent a suflicientquan tity of gas to prevent the pressure from exceeding the selectedlevel.

It is obvious from the foregoing example that the dimensions of thegasholder should be selected so as to provide a reserve capacity of gassuflicient to accommodate the maximum volume change of the gas at theselected pressure which may be expected to result from normaltemperature variations. In a typical example of a floating roof tankhaving a diameter of 120 feet designed for use in a temperature zonewhere daily temperature variations may be as much as 50 F., the amountof gas reserve to be provided in the gasholder is approximately 31 cubicfeet. Floating roof tanks having a smaller diameter or designed for usein other areas Where the temperature variation is not so extreme willrequire a commensurately smaller gas reserve, whereas larger tanks orlarger temperature variations will necessitate a larger gas reserve.

The gas which is introduced through inlet valve 21 may come from any oneof a number of convenient sources, including a replaceable pressurevessel, more commonly called a gas bottle; a pressure vessel commonlycalled an air receiver used in combination with a compressor to maintainthe gas pressure; a vessel containing a liquefied gas such as propane; agas pump consisting of a closed container with inlet and outlet checkvalves arranged to take advantage of temperature changes in thecontainer (for example, the container might be one or more of the closedpontoon spaces of the floating roof); an electrically powered compressoroperated by means of limit switches arranged to operate when thegasholder roof nears its extreme bottom position; or a compressorpowered by the vertical roof movement caused by filling and emptying thetank.

The gasholder shown in the figures is located within a pontoon of apontoon type floating roof. Although this is the preferred embodiment,it is obvious that the gasholder can be positioned elsewhere on thefloating roof, or used in conjunction with a floating roof other thanthe pontoon type, all without departing from the scope of the presentinvention.

The utilization of a gasholder is preferred because it permitstrouble-free and maintenance-free operation of a floating roof having agas inflated seal. Such an arrangement, being almost entirelyself-operating and automatic, does not require the constant attention ofmaintenance personnel that would otherwise be required if the gasholder,which provides an ample reserve of gas at the desired pressure, wereomitted. When a gasholder is used, the only maintenance required is theperiodic checking of the working parts and replenishment of the gassupply. Obviously, however, since the gasholder in the system provides asuificient reserve for daily fluctuations, the demands upon the gassupply are so minor that it would not be necessary to check the gassupply except at infrequent intervals, perhaps once a month. Thismaintenance-free feature of the roof is of particular importance at bulkterminals and marketing terminals, where maintenance and repairpersonnel are not ordinarily employed.

In cases in which the tank owner maintains a close inspection andmaintenance program, as for example in an oil refinery, it is feasibleto eliminate the gasholder from the system and to keep the seal tubeinflated directly from an outside source of gas. Such an arrangement isshown in FIGURE 5, which is a vertical cross-sectional view of anembodiment of the invention which omits the gasholder. In FIGURE 5conduit 31 is connected directly to a supply valve 32 which is selectedof a type to open When the pressure in the tube 14 is less than apredetermined relatively low pressure. The venting valve 33 is also of atype selected to open when the pressure in the tube 14 exceeds apredetermined relatively higher pressure. In this embodiment it isapparent that the normal breathing volumetric changes are accommodatedby the inlet valve 32 and venting valve 33, rather than by the movementof a gasholder root as in the previous embodiment. For this reason agreater amount of gas will be wasted on account of breathing and anequivalently increased amount of gas will have to be supplied from thegas source. This means that a much greater amount of attention isrequired in operating thesystem. If the gas supply consists of pressurebottles of gas or of vessels of liquefied gas, as described above, itwill be necessary to change the bottles or tanks more often than is thecase where a gasholder is employed. Moreover, because both the inletvalve and the venting valve must operate more frequently, themaintenance of these valves becomes more critical. Likewise, in theevent that some type of gas compressor is used as the source of supply,the compressor will be turned on and off much more frequently than isthe case Where a gasholder is employed and the maintenance of this typeof equipment becomes more critical. It will, therefore, be apparent thatthe cost of operating 6 such a system is greater than one in which agasholder is used.

The foregoing detailed description has been given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom, as modifications will be obvious to those skilled in the art.

I claim:

In a cylindrical tank for the storage of a volatile liquid, a floatingroof having a diameter less than the internal diameter of said tank andadapted to float on said liquid, said floating roof having at least onepontoon for providing buoyancy thereto, an annular impervious flexibletube supported by said roof and disposed between said floating roof andthe interior surface of said tank, inflating means for inflating saidtube by gas pressure, pressure-regulating means for maintaining in saidtube a gas pressure Within a predetermined range comprising aconstantpressure, variable-volume gas reservoir located entirely withinsaid pontoon, and conduit means connecting said reservoir with said tubefor gas flow therebetween.

References Cited in the file of this patent UNITED STATES PATENTS1,674,038 Glass June 19, 1928 2,050,685 Wiggins Aug. 11, 1936 2,050,686Wiggins Aug. 11, 1936 2,085,752 Horton et a1. July 6, 1937 2,366,911Laird Jan. 9, 1945 2,538,875 Laird Jan. 23, 1951 2,847,142 McClintock eta1. Aug. 12, 1958 2,888,717 Domitrovic June 2, 1959 3,059,805 Joor Oct.23, 1962

